The present invention relates to a microwave power amplifier, and in particular, relates to a high power transmitter for a satellite communication system.
A transmitter in an earth station for a satellite communication system is required to have high transmitting power, and therefore, the design of a transmitter is an important consideration in constructing an earth station. When a number of signals are transmitted by a single power amplifier, an undesirable intermodulaton occurs due to the non-linear characteristics of the power amplifier which operates close to the saturation output power level, and so, the ratio of the desired signal power to the undersired power is decreased. Further, said intermodulation deteriorates the quality of the reception signal. Accordingly, a large transmitter which is able to provide output power several times as large as the required output power has been used in a conventional earth station so that an undesired intermodulation does not occur.
A prior system for overcoming said intermodulation at the operation level close to the saturation level of a transmitter is a pre-distortion system as shown in FIG. 1, in which numeral 1 is an input signal, 2 is a non-linear circuit, 3 is a power amplifier which is subject to compensation, and 4 is an output signal. It is assumed that the power amplifier 3 has the characteristics as shown in FIG. 1C in which the horizontal axis shows an input power, and the vertical axis shows an output power, the dotted line is the ideal linear characteristics, and the solid line is the actual characteristics. In that case, the non-linear circuit having the inverse characteristics as shown in FIG. 1B, in which the horizontal axis shows an input power, the vertical axis shows an output power, the dotted line shows the linear characteristics, and the solid line shows the characteristics of the non-linear circuit 2, is used. The characteristics of the non-linear circuit 2 is designed so that the combination of the non-linear circuit 2 and the amplifier 3 provides the linear characteristics. The non-linear circuit 2 is constituted by using a diode, or an amplifier as a non-linear element.
Another prior art for compensation of intermodulaton is a high power transmitter is a feed forward amplifier system as shown in FIG. 2, in which the numberal 1 is an input signal, 5 is a divider, 6 is a power amplifier, 7 is a directional coupler, 8 is a combiner, 9 is a variable attenuators, 10 is a subtrarctor, 11 is an auxiliary amplifier, T.sub.1 and T.sub.2 are delay circuits which include phase shifters.
In FIG. 2, the input signal is divided into two parts, one of which is applied to the main amplifier 6. Then, almost all the portion of the output of the amplifier 6 is applied to the combiner 8 through the delay circuit T.sub.1, and some small portion of the output of the amplifier 6 is separated and is applied to the subtractor 10 through the variable attenuator 9. The subtractor 10 also receives the divided input signal through the delay circuit T.sub.2. The delay circuits T.sub.1 and T.sub.2 function to compensate the delay time in the amplifiers 6 and 11, respectively, so that the subtractor 10 and the combiner 8 can receive a pair of inverse-phased signals, and a pair of inphase signals, respectively.
When the input signal level 1 is low and the main amplifier 6 operates at the linear region, no distortion is generated by the amplifier 6. It is supposed that the variable attenuator 9 is adjusted so that the subtractor 10 does not provide an output power when said amplifier 6 does not generate distortion. Then, the output of the subtractor 10 is only the distortion component which is generated by the main amplifier 6 when the input signal 1 is at a high level. The distortion component at the output of the subtractor 10 is amplified by the auxiliary amplifier 11, which provides the amplified distortion component to the combiner 8. The combiner combines the signal which has the distortion from the delay circuit T.sub.1, and the distortion component from the axuiliary amplifier 11, then, the distortions in both the inputs of the combiner 8 are cancelled with each other, and the combiner 8 provides the output signal 4 which is free from distortion.
Although the feed forward system as described in accordance with FIG. 2 has the advantage that the distortion is compensated considerably, the prior feed forward system has the disadvantage that the auxiliary amplifier 11 must have a large power capacity, even equal to that of the main amplifier 6, in order to compensate for the large distortion component. Further, the combiner or coupler 8 is usually a 6 dB coupler, and therefore, the output power of the auxiliary amplifier 11 does not fully contribute to the combined output power 4, and the efficiency of the total system is decreased.
The prior feed forward system has still a further disadvantage that a trouble of a main amplifier makes the system transmitting power one fourth, and if the auxiliary amplifier output power is the same as that of the main amplifier, the system operation becomes too difficult to maintain the communication link.